Power Factor Correction (PFC)
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Lambdaaa Unveils Hyperfast- and Very Soft-Recovery Diodes that Maximize Power Factor Correction Circuit Performance (October 5, 2000)
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The PFC socket is extremely important in high output power switch mode power supplies. The impulse current drawn from the AC line by the bulk capacitor in the input bridge is not only at the fundamental frequency (50 or 60 Hz) but also at integer multiples of it, up to the 20th harmonic (typically odd harmonics) and this causes a worsening of the power factor, typically near 0.7.

PFC Circuit Using Boost Topology
A PFC (Power Factor Corrector) is the solution for all these problems. Typical active PFC implementation employs a Boost topology shown above, which has the following advantages:

  • Small size
  • Output voltage greater than input voltage (universal input);
  • Simple control stage;
  • Input inductance acts like a filter toward AC line;
  • Many control ICs available on the market.
  • PFC of 0.95 or better over line and load variations.

Two types of control are mostly used, discontinuous mode and continuous mode. The main difference is that in the discontinuous mode the MOSFET is turned on only when the inductor current has reached zero, while in continuous Mode the MOSFET is turned on when inductor current is still above zero and therefore all reverse recovery energy must be dissipated in the MOSFET.

Discontinuous mode

This control strategy is very suitable for PFC circuits operating below 200W. The main advantages are extreme simplicity of the control stage and smaller magnetic component. Its large peak currents make it an unsuitable choice above 200W. In discontinuous mode the diode switching losses are negligible and hence it is possible to use fast diodes instead of ultrafast diodes.

The new QuietIR™ family of fast recovery diodes has proved to be particularly suited for this application. Typical devices are EWF…S in D-Pak and 10ETF06 or 20ETF06 in TO-220 and SMD220 when used as boost diodes in discontinous mode circuit topology.

Table 1. Circuit Requirements Fulfilled by QuietIR™ Diodes

Circuit Requirement Solution
Lower Power Conduction losses Lower VF
Less noise Softer Recovery Current Shape
Surge Immunity Improved Robustness

Continuous mode

This type of control is widely used for power levels over 5kW. When the diode is switched off its forward current may be in the order of several amperes and the current slope dIf/dt very high (100 to 250 A/ms). Moreover, when the MOSFET is switched on and the diode begins to switch off, the output loop which is composed of the output capacitor, boost diode, MOSFET and the interconnection parasitic inductances, behaves like a tuned circuit, the resonant frequency of which depends on circuit layout, reverse diode capacitance and output capacitor parasitic parameters. Typical resonance frequencies are around 100MHz and this can cause problems with radiated EMI

In this circuit the boost diode is the most critical component and must be chosen carefully in order to avoid large power losses and EMI.

Table 2. Circuit Requirements Met by the IR Line of UltraFast Diodes

Circuit Requirement Solution
Lower Power Conduction Losses Low VF
Low EMI Low Qrr and Trr

Table 3. Recommended devices are: *ETH06* and K-series MOSFETs. New application-specific diodes and matching HEXFET® power MOSFETs greatly improve PFC circuit performance.

Package trr
IF(avg) Power
8ETH06 TO-220AC 30ns 8A 750W IRFB18N50K
8ETH06S D2Pak 30ns 8A 750W IRFB18N50K
8ETH06-1 TO-262 30ns 8A 750W IRFB18N50K
15ETH06 TO-220AC 35ns 15A 1.0kW IRFB18N50K
15ETH06S D2Pak 35ns 15A 1.0kW IRFB18N50K
15ETH06-1 TO-262 35ns 15A 1.0kW IRFB18N50K
30ETH06 TO-220AC 40ns 30A 1.5kW
30ETH06S D2Pak 40ns 30A 1.5kW
30ETH06-1 TO-262 40ns 30A 1.5kW
30CPH06 TO-247 40ns 30A 1.5kW

IR HEXFRED diodes such as: HFA04TB60, HFA08TB60, HFA15TB60, HFA08TB60 can also be used in this application.